Many radio communication systems are able to operate at several different frequency bands. Hence, when a user terminal is switched on, it has to search for the frequencies that are used in the geographical area where it is located. In theory this could be a time-consuming process, but in practice, it is usually not a problem, since there are normally only a limited number of frequencies to scan for in a particular radio communication system such as e.g. GSM or Bluetooth.
The process is even further simplified by the fact that within a certain geographical area (e.g. a country) there is typically only a subset of frequency bands in use, and the user terminal may be hardwired for those particular frequency bands. Also, when switched on, the likelihood is large that the same frequencies that were used the last time the user terminal was connected to a network are still the ones to use, and the user terminal can start the search with these frequencies for fast results.
There is also a possibility to use a beacon signal that has a fixed frequency in all cases. This is mentioned in the paper “Spectrum Management Methodology” by Andy McGregor. The paper appears in Universal Personal Communications 1993. ‘Personal Communications: Gateway to the 21st Century’. Conference Record., 2nd International Conference on. Publication date 12-15 Oct. 1993, volume 1, on pages 476-479 vol. 1. This paper mentions the use of a control “beacon” which may indicate that the x MHz above or below the frequency used by the control beacon is available for use. In this case the frequency bands are however fixed and the user terminal is aware of the different possibilities of frequency bands, therefore the terminal knows where to look for the beacon channel. The problem appears when the frequency bands are not fixed and the only thing that is known is the spectrum range that has to be scanned in its entirety.
Although the problem of finding the frequencies to use is simple in most traditional systems, usage of very flexible and/or fragmented spectrum potentially to be used in multi carrier systems, may present several difficulties. Systems using multi carrier transmission include e.g. IEEE 802.11, IEEE 802.16, IEEE 802.20, HiperLAN2, Universal Mobile Telecommunications System Long Term Evolution/System Architecture Evolution (UMTS LTE/SAE)) and radio interface proposals such as Wireless World Initiative New Radio (WINNER) concept. In a multi-carrier system the transmission bandwidth, i.e. the carrier is divided into a number of sub-carriers, which are typically arranged to be orthogonal or near orthogonal. The signals modulated on the subcarriers can thus be transmitted in parallel. There are two main reasons why the number of candidate carriers to search for may be large in such future systems: Firstly, due to possible regulatory requirements on spectrum flexibility, the total frequency range where such a system may operate may be very large, perhaps 1-6 GHz or even wider; Secondly, one advantage of multi-carrier systems is that it is simple to vary the system bandwidth by activating different number of sub-carriers. This means that it is important to establish where in the wide frequency range the system operates, but also to establish the actual bandwidth used.
There are potentially a large number of possible bandwidths. Since the multi-carrier system may be designed to utilize large and small unused “holes” in the radio spectrum, there will not necessarily be any specific frequency slots for a given bandwidth. For example, 10 MHz carriers will not necessarily be found only on frequencies that are integer multiples of 10 MHz. The situation is further complicated if fragmented spectrum is employed, i.e. if the spectrum used is composed of two or more frequency ranges. NB it may be sufficient that one of these frequency ranges are detected to be able to identify the network.
The following examples show that when there are many possible combinations of bandwidths and many possible locations of the carrier, the search effort will be very time consuming. Assuming 6 possible carrier bandwidths {2.5 MHz, 5 MHz, 10 MHz, 20 MHz, 50 MHz, 100 MHz} and 5000 possible locations of the carrier within 1-6 GHz and a spacing of 1 MHz, there are in total 6×5000=30000 search candidates. With a candidate test time interval of 10 ms, the worst-case time for identifying the network could be about 300 seconds, which may be deemed unacceptable. When the number of combinations are considerably smaller, as in most current systems, this is not a significant problem.
The search needs not necessarily only be performed when the user terminal is powered up. With a multitude of both licensed spectrum for wide area coverage and license-free spectrum for private local use, e.g. indoors, for personal area networks etc), a new scan may have to be performed very often in order to ensure optimal interruption-free connection (e.g. when entering a building where there is no outdoor-to-indoor coverage, but only a private hotspot).